Because everyone knows that fists act on their own accord.
"It has nothing to do with you."
The words resonated in her head although she saw them forming in his lips as soon as he had decided to say it. Her own words before that shocked him well enough; that much was plain to see. It was written all over his face.
He had been doing well up until about there. Ichigo had been keeping up his façade for a few months now, hadn't he? Tatsuki wasn't able to pinpoint the exact date he started to behave strangely; it might have had to do with that truck accident that happened to his house. She also knew that ever since the abnormality, Orihime had become slightly distant.
That much alone was cause for concern.
Then Tatsuki's sightings started. If epic battles in her home-town that included classmates weren't an indication, she didn't know what was. Ichigo's face told her she wasn't supposed to know. She didn't need the words to confirm that.
'It has nothing to do with you.' Like an out of control train the sentence charged into her stomach.
Fists react on their own. It's a natural law of science: Newton's law or something like that. Arisawa's fist is no exception. In fact, it is a perfect example.
She felt her knuckles collide with his jaw, in turn shattering the school's second floor window. The school will want her parents to pay for that, she thought in the back of her mind.
She was dimly aware of Ichigo's stupid friends rushing to hold her back, and she knows she is shouting about nakama or something like that. She believed it, too. And wanted to keep on believing it. She wanted to know that Ichigo is her friend and that he could trust her. But more than that, she needed to know where Orihime was. She eventually fell silent, the torrent of force and feeling momentarily muffled as she ran out of steam.
"...Don't concern yourselves with me."
Keigo's face looked a bit funny and Tatsuki pulled out of his grip and slowly stood on her own, watching the back of the kid she once thought she knew. The least he could have done was let her know what was going on so she could verify Orihime's safety, but she had a feeling he wouldn't comply.
He has a real hero complex, Tatsuki mused. She knew full well that he was trying to protect his friends again.